EP1921738A2 - Method and device to control a dc-ac converter, particularly for a wind energy plant - Google Patents

Abstract

The method involves driving a generator (3) of a converter by a wind rotor (2), for supplying electrical power into a middle voltage power line during voltage drop in the power line, where the converter has an intermediate circuit (42) with an energy storage (43). A low value of the stored energy is determined in dependent of the capacity of the energy storage and lowering the energy stored in the energy storage such that peak power that occurs at the end of the sudden voltage disturbance in the intermediate circuit can be received in the energy storage.

Description

The invention relates to a method and apparatus for operating an inverter, in particular for wind turbines with a driven by a wind rotor generator for feeding electrical power into a network, wherein the inverter has a link with an associated energy storage. More specifically, it is important to lower the stored energy in the energy storage and to feed power from the intermediate circuit in the network at a sudden voltage failure, in particular voltage drop in the network.

Modern wind turbines are usually variable in speed, so that the generator generates alternating current with different frequency. For feeding into a fixed frequency supply network (usually 50 Hz) a frequency conversion is required. For this purpose frequency converters are used. Conventionally, wind turbines have been operated so that they disconnect from the grid in case of network errors, especially short circuits. However, with the increasing prevalence of wind turbines and the increase in installed wind power, grid-supporting operation of the wind turbines, in terms of frequency and voltage, is instead required by the grid operators.

Basically, there are two main aspects that are taken into account when fulfilling the requirements of the network operator Need to become. The first aspect is that in the event of a sudden change in the line voltage, an increase in the voltage in the DC link must be limited in order to prevent the converter from switching off. The second aspect is that the mechanical load of the drive train leading from the wind rotor to the generator should be reduced as far as possible in terms of height and frequency as a result of step-like mains voltage changes.

In the first aspect, it is known to arrange a special circuit between the generator and the converter, which limits the voltage increase in the intermediate circuit ( WO-A-03/065567 ). It is further proposed to limit the increase in voltage with a generally present anyway DC link chopper. Alternatively, it can also be provided to reduce the intermediate circuit voltage and the losses by reducing the switching frequency of the converter, and thus to increase the converter current ( WO-A-2005/027301 ).

It has been found that the known measures are effective only with respect to one of the two aspects, but not with respect to the other. Thus, the known measures to reduce the voltage increase usually lead to an increase in the loads of the drive train, and vice versa.

Based on the last-mentioned prior art, the object of the invention is to provide a method and a device for operating an inverter, with which a better behavior is achieved.

The solution according to the invention lies in a method and in a device according to the features of the independent claims. Advantageous developments are the subject of the dependent claims.

In a method for operating an inverter, in particular for wind turbines with a generator driven by a wind rotor for feeding electrical power into a network even during a sudden voltage disturbance, in particular voltage drop in the network, wherein the inverter comprises a DC link with an associated energy store, is according to the invention provided that a low value to which the energy is lowered in the intermediate circuit, is determined in such a manner depending on the capacity of the energy storage, that a resulting at the end of the sudden voltage disturbance in the intermediate circuit peak power is absorbed in the energy storage.

The idea of the invention is to reduce the energy stored in the intermediate circuit to such an extent that the activation of protective devices of the converter, such as, in particular, a DC link chopper or a crowbar between generator and converter, occurs as far as possible in the event of a step voltage failure can be. The invention achieves this by deliberately lowering the voltage (or the current) in the intermediate circuit, more precisely in its mostly capacitive or inductive energy store, in order to free sufficient storage capacity depending on the magnitude of the voltage jump, so that the voltage returns at the end of the step Voltage disturbance occurring power peaks can be recorded. The risk of exceeding critical limit values in the DC link can thus be counteracted. The invention thus achieves a reduction in the frequency and duration of the Activation of inverter protection circuits. This not only improves the quality of the electrical power fed into the network, but in particular avoids harmful torque fluctuations, and thus avoids the resulting from the torque fluctuations load or risk of damage to components such as the generator and its drive train. In addition, the invention improves the "ride-through" property with temporary break-in and return of the mains voltage, especially in the event of sudden mains voltage disturbances.

The term sudden voltage disturbance encompasses both a sudden reduction of the voltage in the network, a short circuit being the extreme case, as well as a sudden increase beyond the tolerance range provided for normal operation.

A peak power is understood to mean a transient, in particular induced, overvoltage or a transient, in particular induced, overcurrent.

Although it is known to lower the voltage in the DC link in the event of a short circuit ( WO-A-2005/027301 ), however, this was done solely with the aim of providing more favorable operating conditions on the inverter for the supply of electrical power during the period of the voltage drop into the grid. The conditions on return of the mains voltage was taken no consideration. In particular, it was not considered whether the voltage in the DC link was reduced so much that so much energy could be stored in the energy store of the DC link, in order to return to the mains voltage to have a sufficient reserve available and thus to prevent ignition of the crowbar circuit as possible. Rather, it was dependent on random factors whether the voltage return was so slow or so sudden that the overcurrents occurring in the intermediate circuit could be buffered or not. The invention now ensures that as much energy absorption capacity as possible is provided in the DC link until the mains voltage returns. This can be achieved in a surprisingly simple way, a significant improvement in the performance of the inverter and thus the entire wind turbine. It not only improves the operating behavior, but it is also a protection of the components of the wind turbine reached.

In general, the method is performed so that the inventive lowering to the low value during the sudden mains voltage failure occurs in preparation for the return of the mains voltage. Since the time until the return of the mains voltage is usually not known, the reduction according to the invention to the low value expediently takes place immediately after detection of the mains voltage fault. However, it can also be provided that the lowering takes place in preparation for a mains voltage disturbance occurring at a later time. This preemptive variant offers the advantage that free capacity is already available at the beginning of the power failure to absorb overcurrents. Preferably, this is done so that the lowering is performed for the first time in preparation for the expected mains voltage failure and a second (or further) time is performed after the occurrence of the mains voltage fault has been detected. Thus, the benefits of the invention extend not only to the return of the Mains voltage, but can already come at the beginning of the power failure to unfold.

According to a particularly preferred embodiment, the inventive method is used in a wind turbine with a double-fed generator. Conventionally, it is precisely this type of plant that causes great difficulties, since the stator / rotor winding pair acts as a transformer and thus considerably higher voltages are induced in the rotor circuit than in the rest of the network. As a rule, limit values for the output voltage of the inverter are exceeded. Short-circuit-like currents in the rotor and stator can occur as transients. This can result in unwanted shutdown of the machine side inverter. In addition, as a result of the high currents in the rotor, a real power is supplied to the intermediate circuit for a short time, which is above the rated power, whereby the voltage in the DC link increases particularly quickly and sharply. Correspondingly large were the switching frequency and intensity of the converter protective circuits and the resulting negative consequences for the power quality and the mechanical load of the drive train. These adverse effects can be reduced thanks to the application of the invention to this type of plant.

mit U_Netz als Effektivwert der verketteten Netzspannung, N der Drehzahl, N_Sync der Synchrondrehzahl, ω_21Gener als Windungszahlverhältnis Rotor zu Stator des Generators und K_Last als lastabhängigen Faktor.The low value to which the energy in the intermediate circuit is lowered is expediently calculated according to the following formula: $$U_{\mathit{DC\; min}}=\mathit{Max}\left(U_{\mathit{network}}\cdot \sqrt{2}\cdot k_{\mathit{load}1}.U_{\mathit{network}}\cdot \sqrt{2}\cdot \frac{N-N_{\mathit{Syn}}}{N_{\mathit{Syn}}}\cdot {\omega }_{21\mathit{Gener}}\cdot k_{\mathit{load}2}\right)$$

with U _network as rms value of the interlinked mains voltage, N of the speed, N _{sync of} the synchronous speed, ω _21Gener as the number of turns rotor to stator of the generator and K _load as load-dependent factor.

The lowering of the energy level in the intermediate circuit to the low value can take place in various ways. This is preferably done by driving the grid-side inverter, so as to dissipate the excess energy in the intermediate circuit useful in the network. If this is not desired or this is not possible, since there is no more mains voltage, then alternatively the chopper circuit can be driven until the desired energy value is reached. This procedure makes use of the fact that a chopper circuit is usually already present for component protection anyway. It can be used to deliberately lower the energy level in the DC link. Additional hardware effort then does not need to be incurred.

The inverter with intermediate circuit usually has a structure in which a generator-side inverter is connected via the intermediate circuit with a network-side inverter. This design has the advantage that it allows a power flow in both directions, and is therefore particularly suitable for double-fed asynchronous machines as a generator. However, it should not be ruled out that the generator-side inverter is designed as a rectifier. If the following is the network-side inverter, so it may also be meant a rectifier.

To explain the invention, it is further assumed that, as usual, a crowbar circuit for the protection of the Generators and the inverter is provided. The term crowbar circuit is to be understood here in general, it is not limited to the most common embodiment as a kind of short circuit.

Conveniently, after the occurrence of the voltage dip, the network-side inverter is given a voltage setpoint which is above the amplitude value of the broken-down mains voltage. Preferably, the voltage setpoint is at most 15%, more preferably between 5% and 10%, above the amplitude value of the buried power supply voltage. This activation of the inverter ensures that massive current from the DC link can be fed into the grid, with a low voltage setting in the DC link depending on the voltage setpoint.

However, it can be provided that the generator-side inverter is deactivated during the voltage disturbance until the voltage in the network has again reached a predetermined value of the mains nominal voltage. This procedure not only has the advantage that the likelihood of overshoots of critical limits in the DC link is further significantly reduced. It also has the advantage that, depending on the magnitude of the voltage jump, only little power flows from the generator into the DC link, so that only correspondingly less power needs to be dissipated to lower the energy level in the DC link to the low value. This is particularly favorable if the step-shaped voltage disturbance is a voltage increase. Regardless, it is expedient to reconnect the generator-side inverter after a minimum time, so can be compensated by activating the crowbar compensating operations.

Conveniently, a Gleicnspannungszwischenkreis with a capacitor used as an energy store as a DC link. But this is not mandatory. If desired, it can also be provided that the intermediate circuit is designed as a DC intermediate circuit with a coil as energy storage.

The invention further relates to a device having the features of the independent device claim. It comprises as a core a setpoint module which is provided for adjusting the level of energy stored in the intermediate circuit. For further explanation and for further advantageous embodiments, reference is made to the above statements.

Fig. 1

eine schematische Übersichtsdarstellung einer an ein elektrisches Versorgungsnetz angeschlossenen Windenergieanlage gemäß der Erfindung; und

Fig. 2

eine Schaltungsansicht eines Umrichters der Windenergieanlage in Figur 1;

Fig. 3

ein Diagramm mit Spannungszeitverläufen bei einem Spannungseinbruch im Netz ohne die Erfindung; und

Fig. 4

ein Diagramm mit Spannungszeitverläufen bei einem Spannungseinbruch im Netz gemäß der Erfindung.

The invention will be described below with reference to the accompanying drawing, in which an advantageous embodiment is shown. Show it:

Fig. 1

a schematic overview of a connected to an electrical supply network wind turbine according to the invention; and

Fig. 2

a circuit view of an inverter of the wind turbine in Figure 1;

Fig. 3

a diagram with voltage waveforms at a voltage dip in the network without the invention; and

Fig. 4

a diagram with voltage waveforms at a voltage dip in the network according to the invention.

FIG. 1 shows schematically a wind energy installation designed for carrying out the invention, which is provided with the reference number 1. The wind turbine 1 has, in a known manner, a nacelle 11 pivotally mounted on a tower 10. At the front side of a wind rotor 2 is rotatably arranged. It drives via a rotor shaft to a generator 3, which is preferably designed as a double-fed asynchronous machine with multi-stranded rotor and stator winding. The stator winding of the generator 3 is connected directly to a multi-strand connecting line 18 of the wind turbine 1. The rotor winding (not shown) is connected to a (generator side) inverter 41 of an inverter 4; parallel to the rotor winding, a crowbar circuit 40 is connected as a converter protection circuit. With its (network-side) inverter 48 of the inverter 4 is connected to the connecting line 18. Further, a controller 5 is provided for the operation of the inverter 4.

The electrical power generated by the generator 3 is conducted via the connection line 18 to a transformer 19. It is designed to implement the voltage supplied by the wind energy installation 1 from the low-voltage level to the voltage level of the medium-voltage network 9. The transformer 19 can transmit both the power of a single wind turbine 1 or the power of a plurality of wind turbines (wind farm).

Figure 2 shows the schematic representation of the inverter 4. It has as main components a generator side Inverter 41, a network-side inverter 48 and a capacitive intermediate circuit 42. The inverters 41, 48 of the inverter 4 are formed with self-extinguishing semiconductor switches (in the illustrated example, IGBT 411, 481 with a blocking voltage of 1700 V) plus anti-parallel reverse current diodes 412, 482. In this embodiment, an active power transmission in both directions is possible, wherein both inverters 41, 48 can additionally and independently of one another exchange reactive power with the generator or the network. The control of the IGBT 411, 481 is carried out by the controller 5 in a conventional manner, so that need not be discussed in detail at this point.

The intermediate circuit 42 is formed as a DC voltage intermediate circuit. It comprises a plurality of capacitors in parallel and series connection (in the example with a nominal voltage of 1100 V) as energy store 43. It should be noted that coils are provided in accordance with a DC intermediate circuit.

The intermediate circuit 42 is also associated with a DC link chopper 45. It serves to reduce energy stored in the intermediate circuit 42 in a controlled manner. It comprises an IGBT 451 and an antiparallel-connected protective diode 452, which are connected in series with a load resistor 453 having a load inductance 454. When activating the intermediate-circuit chopper 45, the energy contained in the intermediate circuit 42 is stored substantially in the energy store 43 is diminished quickly. An increase of the voltage in the intermediate circuit 42 to supercritical values can thus be counteracted effectively. As a result, the DC link voltage can then be limited if briefly high active power increases from the generator-side inverter 41 pending, which can not be absorbed by the network-side inverter 48 at the same rate of increase. This is the case in particular in the event of sudden changes in mains voltage. If the power of the intermediate circuit chopper 45 is also insufficient, in order to cover the momentarily present differential active power of both inverters 41, 48, the crowbar 40 is activated in a manner known per se. It prevents the supply of active power from the generator-side inverter by short-circuiting the rotor winding.

For the control of the DC link chopper 45, a special control module 51 is formed in the controller 5. Its function will be explained in more detail below. It outputs at its output switching pulses for the intermediate circuit chopper 45, so that this turns on in dependence on the output signal (ignites) or not.

The setpoint module 51 has an input to which a voltage measuring device 50 for the voltage in the intermediate circuit 42 is connected. It also has an optional input at which a value for a desired pause time is adjustable. Pause time is understood here to mean a time period which should elapse until the converter 4, more specifically the network-side inverter 48, feeds power back into the network at the end of the step-like voltage disturbance. This period of time is referred to as T _s . For them an empirical value (for example, 10 milliseconds) can be used, which is possibly firmly implemented. As a period of time, a so-called orientation time can be used, which indicates the period of time that the inverter 48 needs to get back to the Synchronize network. The set-point module 51 is designed to generate a charge or discharge current from the rotor 3 into the intermediate circuit 42 of the converter 4 as a function of the time duration and an overcurrent occurring at the end of the step-like voltage failure (eg return of the mains voltage after break-in or short-circuit) to calculate an amount of energy to be received in the intermediate circuit 42. So that this amount of charge / energy can be absorbed without exceeding critical limits in the intermediate circuit 42, the setpoint module 51 determines a low value to which the energy storage 43 of the intermediate circuit 42 is discharged after the beginning of the voltage disturbance, so that there is still enough capacity to absorb the voltage at power recovery occurring charge or energy remains. The setpoint module 51 is further configured to drive the intermediate circuit chopper 45 in such a way that upon detection of a step-like voltage disturbance in the network 9, the energy store 43 of the intermediate circuit 42 is discharged to the low value. In the capacitive intermediate circuit 42 shown in the example, this is done by lowering the intermediate circuit voltage. The capacitors of the energy storage 43 are thereby discharged to the extent that additional active power can be absorbed for a limited time. Without the inventive discharge to the low value this type of power storage could not be done. This achieves, on the one hand, to relieve the DC link chopper 45 and, on the other hand, the probability of activating the crowbar circuit 40 can be reduced. The risk of harmful vibrations in the drive train, in particular due to fluctuations in the electric braking torque by igniting the Crowbar circuit 40 at the end of the surge voltage disturbance and return of Mains voltage in its normal range, can thus be counteracted thanks to the invention.

It does not matter whether the full nominal voltage has already been reached or not at the conclusion of the voltage recovery and the decay of the resulting overcurrents in the intermediate circuit 42. Once it has been reached, the grid-side inverter 48 can immediately begin to feed electrical power into the grid 9. If not yet reached, the intermediate circuit 42 charges in a conventional manner via the rectifier 41 until the nominal voltage (typically 1100 volts) is reached, and the network-side inverter 48 then feeds power into the network 9 in a normal manner.

FIGS. 3 and 4 show time profiles of the voltages in the network and in the intermediate circuit as well as the activity of the crowbar circuit 40 in the event of a mains voltage dip as a step-shaped voltage disturbance. It is assumed that at the time t = 0.1 seconds, the mains voltage abruptly breaks by a value of about 30% of the nominal voltage. About 150 milliseconds later, at time t = 0.25 seconds, the line voltage returns. The course of the mains voltage is shown in an upper curve a) in FIG. By means of a voltage detector integrated in the control device 5, a corresponding break-in signal can be generated (see curve b). In the curve c), the actual voltage in the intermediate circuit 42 is shown by a solid line. It can be seen that a slight increase in voltage occurs at the beginning of the voltage dip. This voltage increase would continue and could eventually lead to component damage unless the crowbar circuit 40 has been fired for protection (see Curve d). Thanks to the activity of the crowbar circuit 40, an excessive voltage increase in the DC link is thus counteracted. The voltage in the intermediate circuit 42 decreases until it finally reaches a value slightly below the desired value and shuts off the crowbar circuit. The voltage in the intermediate circuit 42 can then slowly rise again to its desired value (dashed curve c). A arranged in the intermediate circuit 42 energy storage is also charged accordingly to a normal energy value. This is a quasi-stationary state is reached, which can continue until the end of the voltage dip. If the mains voltage now returns (t = 0.25 seconds, see FIG. A), as explained above, a high voltage is induced in the rotor circuit of the doubly-fed generator. The voltage in the intermediate circuit 42 rises accordingly (see curve c), until finally the crowbar circuit for the protection of the components from overloading ignites (see curve d). The actual voltage in the intermediate circuit 42 then drops due to the activation of the crowbar circuit until it again reaches a value slightly below the desired voltage and the crowbar circuit shuts off. With the return of the mains voltage, the detection signal for the voltage dip is reset accordingly (see curve b).

FIG. 4 shows corresponding time profiles for the voltages and signals in a device and a method according to the invention. Again, at the time t = 0.1 second breaks the mains voltage (curve a). The actual voltage in the intermediate circuit 42 increases accordingly, so that the crowbar circuit 40 ignites for protection against harmful overvoltage. The detection signal output from the voltage drop detector circuit is thereby shown in curve b). The course of the voltage in the intermediate circuit initially corresponds to that shown in FIG. The voltage first increases until the crowbar circuit 40 fires and eventually shuts off when the actual voltage (solid line) in the DC link 42 has dropped to a value below the original target voltage. On the basis of the detected voltage drop (curve b), the setpoint module 51 according to the invention determines a lower energy level for the intermediate circuit 42. In the exemplary embodiment shown, this is a voltage intermediate circuit, so that a setpoint voltage (dashed curve) is output as the output value (see dashed curve) c). It amounts to only slightly more than 60% of the original voltage in the intermediate circuit 42. This means that the stored energy in the energy storage 43 of the intermediate circuit 42 is lowered to a low value, which is only about 36% of the original value. Thus, in the energy storage 43, a considerable capacity is free to absorb overcurrents occurring in the intermediate circuit 42 when the mains voltage returns. During the continuation of the voltage drop in the network, the actual voltage in the intermediate circuit (solid curve c) gradually decreases to the low value for the setpoint voltage (dashed curve c) output by the reference module 051. At the moment of voltage return in the network (at t = 0.25 seconds), the voltage in the intermediate circuit 42 rises rapidly because of the voltage ratio in the rotor circuit of the doubly fed generator. Since the energy store 43 is discharged to its low value, its free capacity can be used to absorb as much of the introduced by the voltage jump in the intermediate circuit 42 energy in the energy storage 43. This has enough free capacity in the energy storage 43 the desired effect that the actual voltage in the DC link 42 at power recovery, although it does not exceed the original target level. Ignition of the crowbar circuit 40 can thus be avoided in many cases. In the further course, the actual voltage in the intermediate circuit 42 gradually approaches the original value, wherein the setpoint is again gradually increased to the nominal setpoint. The inverter 4 can then go back to normal operation and feed energy into the electrical grid.

Thanks to the lowered to a low reference voltage in the intermediate circuit 42 of the capacitor is discharged as energy storage 43 in the DC voltage intermediate circuit 42 so far that he can absorb the occurring over-current overcurrents wholly or at least in part, depending on the size of the voltage dip. An ignition of the Crowbar on return of the mains voltage can thus be counteracted. Unwanted stresses due to torque fluctuations on the generator 3 and the drive train leading to the wind rotor 2 can be reduced thereby.

For the operation of the invention, it does not matter whether the machine-side inverter 41 in the initial phase of the voltage dip immediately after the activity of the crowbar circuit 40 dies again feeds electrical power into the DC link 42, or if the re-feeding is delayed until the mains voltage has again reached a defined higher value (of, for example, 90% of the nominal mains voltage). A delay has the advantage that the overcurrents occurring during the return of the voltage and the resulting supply of the intermediate circuit 42 via the return current diodes 412 are lower, so that the DC link chopper 45 is only slightly loaded and therefore can be small in size.

Finally, it should be noted that the own DC link chopper 45 is optional for the invention. The removal of energy from the intermediate circuit 42 in accordance with the invention in order to achieve a low value can be achieved by suitable activation of the network-side inverter 48 (this case is based on the illustration in FIG. 4). Although this presupposes that there is still a certain residual voltage in the network, this is usually met in the case of a wind turbine connected via a medium-voltage transformer 19, even in the event of a short circuit. - It can also be provided to dimension the intermediate circuit chopper 45 stronger, so that it can also act as a crowbar circuit 40. In this case, the external crowbar circuit 40 is dispensable. The effort is reduced. It should be noted, however, that in this case at least the reverse current diodes 412 of the generator-side inverter 41 must be dimensioned correspondingly large.

The operation according to the invention can be summarized as follows. Depending on an operating point of the mains voltage and the rotational speed of the generator, the lowest possible nominal value is determined and set as the low value for the energy in the DC link. As a result, a significantly reduced amount of energy of the DC link compared to the nominal value is set in many operating points - especially after the mains voltage. By a jump in time change in the mains voltage transient power is introduced into the DC link and can be partially corresponding to the previously reduced amount of energy of the DC link are absorbed by the energy storage. In particular, lowering the DC link voltage from the nominal value during a line fault - in preparation for the return of the mains voltage - causes a reduction in the frequency of the inverter protection circuit activity, depending on the level and rate of rise of the line voltage. The amount of the mechanical drive train load of the wind turbine as a direct or indirect consequence of sudden changes in mains voltage is thereby reduced or the frequency of the mechanical loads reduced, in particular after voltage changes with low altitude and / or low slew rate.

Claims (18)

Method for operating an inverter, in particular for wind energy installations with a generator (3) driven via a wind rotor (2), for feeding electrical power into a network (9) even during a sudden voltage disturbance, in particular voltage drop in the network (9), wherein the converter (4) has an intermediate circuit (42) with an associated energy store (43), characterized by determining a low value of the stored energy as a function of the capacity of the energy store (43) and lowering the stored energy in the energy store (43) such that a at the end of the sudden voltage disturbance in the intermediate circuit (42) resulting peak power in the energy storage device (43) is receivable. A method according to claim 1, characterized by lowering the energy level by driving a DC chopper (45) until the low value is reached. Method according to Claim 1 or 2, characterized by predetermining a voltage setpoint for a line-side inverter (48) of the converter (4), which causes a discharge of the energy store (43) to the low value. A method according to claim 3, characterized by selecting the voltage command value such that it lies above the voltage in the network (9) when the voltage drops preferably at most 15%. Method according to one of claims 1 to 4, characterized by a reconnection of the generator-side inverter (41) when the low value in the intermediate circuit (42) is reached. Method according to Claim 5, characterized by restarting only after a minimum period of time has elapsed since the activation of an inverter protection circuit (40). Method according to one of Claims 1 to 4, characterized by delaying the connection of the generator-side inverter (41) until the voltage in the network (9) has reached a predetermined value of preferably 40-80% of the nominal mains voltage. Method according to one of claims 1 to 7, characterized by using a DC voltage intermediate circuit (42) with a capacitor as an energy store (43). A method according to claim 8, characterized by calculating the low value as a voltage according to the formula $$U_{\mathit{DC\; min}}=\mathit{Max}\left(U_{\mathit{network}}\cdot \sqrt{2}\cdot k_{\mathit{load}1}.U_{\mathit{network}}\cdot \sqrt{2}\cdot \frac{N-N_{\mathit{Syn}}}{N_{\mathit{Syn}}}\cdot {\omega }_{21\mathit{Gener}}\cdot k_{\mathit{load}2}\right)\mathrm{,}$$

Method according to one of claims 1 to 7, characterized by using a DC intermediate circuit with a coil as energy storage. Method according to one of claims 1 to 10, characterized by detecting a sudden voltage disturbance and then determining and lowering the low value. Method according to one of claims 1 to 10, characterized by determining and lowering to the low value before detecting the sudden voltage disturbance. A method according to claim 12, characterized by re-determining and lowering after detecting the sudden voltage disturbance. Inverter, in particular for wind energy installations with a generator (3) driven via a wind rotor (2) for feeding electrical power into a network (9) even during a sudden voltage disturbance, in particular voltage drop in the network (9), wherein the converter (4) has a Having intermediate circuit (42) with an energy store (43), and a control device (5) is provided, characterized in that the control device (5) has a setpoint module (51) for setting an energy level in the intermediate circuit (42) which is adapted thereto in that a low value is determined in dependence on a capacity of the energy store (43) in the intermediate circuit (42) in such a way that a peak in power occurring at the end of a sudden voltage disturbance can be absorbed in the energy store (43). Converter according to claim 14, characterized in that the setpoint module (51) is designed such a voltage setpoint for a network-side inverter (48) of the inverter (4), which causes a discharge of the energy store (43) to the low value. Converter according to claim 14 or 15, characterized in that the intermediate circuit (42) is a DC voltage intermediate circuit with a capacitor as an energy store (43). Converter according to claim 16, characterized in that the low value is a voltage according to the formula $$U_{\mathit{DC\; min}}=\mathit{Max}\left(U_{\mathit{network}}\cdot \sqrt{2}\cdot k_{\mathit{load}1}.U_{\mathit{network}}\cdot \sqrt{2}\cdot \frac{N-N_{\mathit{Syn}}}{N_{\mathit{Syn}}}\cdot {\omega }_{21\mathit{Gener}}\cdot k_{\mathit{load}2}\right)$$

is. Converter according to claim 14 or 15, characterized in that the intermediate circuit is a DC intermediate circuit with a coil as energy storage.

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